1. Field of the Invention
The present invention relates to a wireless power transmission audio system for transmitting wirelessly an audio signal and electric power that drives a loudspeaker and also relates to a device on the transmitting end and a loudspeaker for use in such a system.
2. Description of the Related Art
Recently, as cellphones, terrestrial digital TV broadcasting and their related technologies have been further advanced, wireless receivers that receive text data, audio data and telecasts wirelessly as radio waves without using a cable connection have become more and more popular. Currently, electric power is still supplied through a cable to most of those wireless receivers in order to charge its built-in battery to the point that the device is ready to use. However, as those wireless telecommunications technologies have been developing, a lot of people have been attempting nowadays to transmit electric power, as well as those data signals, wirelessly, too.
For example, wireless power transmission technologies by electromagnetic induction have already been applied to various consumer electronic products such as electric shavers and motorized tooth brushes, and have increased their handiness for general consumers successfully. Meanwhile, techniques for transmitting electric power and signals at the same time have also been developed. For example, Japanese Patent Application Laid-Open Publication No. 2004-336513 (which will be referred to herein as “Patent Document No. 1” for convenience sake) discloses a wireless power transmission scheme for a loudspeaker that adopts the electromagnetic induction method.
Patent Document No. 1 discloses a scheme for transmitting not only electric power but also an audio signal from the power transmitting end to the power receiving end via electromagnetic induction. Specifically, as shown in FIG. 18, a transmitting coil 911 for a transmitter 901 (which may be an audio device) and a receiving coil 912 for a receiver 2 (which may be a loudspeaker) are arranged to face each other with a bathroom wall 99 interposed between them. In such an arrangement, those coils are electromagnetically coupled together through the wall 99 and electric power and an audio signal are transmitted wirelessly to the loudspeaker 902. With such an arrangement, only the loudspeaker 902 may be put in the bathroom with the audio device itself such as a CD player left outside of the bathroom. Consequently, it is possible to avoid an unwanted situation where condensation produced in the audio device such as a CD player prevents the CD player from retrieving the audio signal requested.
According to Patent Document No. 1, the transmitting coil 911 and the receiving coil 912 are fixed on two opposite sides of the wall, and the distance between those coils is constant. In other words, the distance between the device 901 and the loudspeaker 902 is constant. However, when electric power and an audio signal are transmitted wirelessly to a loudspeaker, the distance from the audio device to the loudspeaker is not always constant but the position of the loudspeaker may need to be changed arbitrarily in some system. In that case, the distance from the audio device to the loudspeaker varies according to how the user will use that system. If such a wireless power transmission method by electromagnetic induction is adopted, it is difficult to set the distance between the transmitting and receiving coils 911 and 912 to be long enough. This is because according to the electromagnetic induction method, the longer the coil-to-coil distance, the more significantly the power transfer efficiency would decrease. For that reason, such a wireless power transmission system is supposed to be used with the coils always arranged close to each other, and therefore, is hard to cope with a situation where the coil-to-coil distance varies.
Wireless power transmission is not always carried out by such an electromagnetic induction method but may also be done by magnetic resonance method. And devices that uses the latter method have been developed in increasing numbers lately. According to the magnetic resonance method, a capacitor is connected in series or in parallel to each of a transmitting coil and a receiving coil to form two resonant circuits, and electric power is transmitted between those two resonant circuits via a magnetic field. With such a technique adopted, power can be transmitted between two devices that are arranged at even more distant locations. Thus, people hope that such a technique will be applicable to the audio system described above.
To always achieve high transfer efficiency by the magnetic resonance method, the impedance of the transmitter (i.e., on the power supply side) as viewed from the resonant circuit on the transmitting end and the impedance of the receiver (i.e., on the load side) as viewed from the resonant circuit on the receiving end should be controlled appropriately. Suppose a capacitor is connected in series to each of the transmitting and receiving coils 911 and 912 to make each of those coils operate as a resonator. In that case, if the impedance value is maintained at a constant value A as shown in FIG. 19B, then a maximum transfer efficiency B is achieved at a particular distance x as shown in FIG. 19A. That is why it is not true that the shorter the distance between the transmitting and receiving coils 911 and 912, the higher the transfer efficiency.
Unless the impedance values are controlled, the transfer efficiency will decrease even if the distance is shorter than x as shown in FIG. 19A. That is why to achieve high transfer efficiency in a situation where the coil-to-coil distance has varied, the impedance of the transmitter as viewed from the resonant circuit on the transmitting end and that of the receiver as viewed from the resonant circuit on the receiving end need to be controlled appropriately. The same can be said even when power should be transmitted wirelessly to the loudspeaker. That is to say, to maintain high power transfer efficiency, it is important to vary the impedances according to the distance between the transmitting and receiving coils 911 and 912.
FIGS. 20A and 20B show how the transfer efficiency changes with the distance between the transmitting and receiving coils if the impedance of the transmitter as viewed from the resonant circuit on the transmitting end and that of the receiver as viewed from the resonant circuit on the receiving end are controlled appropriately. As shown in FIG. 20B, if the impedances on both of the transmitting and receiving ends are controlled, the shorter the distance, the larger the best impedance value z tends to be as indicated by the solid circles. That is why by controlling the impedance according to the distance so that the impedance becomes as close to the best value as possible, even if the distance is not x, the transfer efficiency can also be kept higher as indicated by the solid circles in FIG. 20A than in a situation where the impedance is not controlled but fixed as indicated by the open circles. Furthermore, as indicated by the solid triangles in FIGS. 20A and 20B, even by controlling the impedance on only one of the two sides (e.g., on the transmitting end in the example illustrated in FIGS. 20A and 20B), it is also possible to prevent the transfer efficiency from decreasing. As can be seen from these results, to achieve high transfer efficiency even when the distance varies, it is important to carry out impedance control (i.e., impedance matching).
If impedances are controlled, the impedances should be controlled continuously according to the distance using a variable inductor and a variable capacitor in order to maintain as high transfer efficiency as possible. Nevertheless, a circuit for changing the impedance continuously usually has a large circuit size and requires a complicated control. That is why to overcome such problems, instead of performing such a continuous control, circuit elements (such as capacitors and inductors) with mutually different fixed impedances and multiple switches may be provided. And if the distance has varied to a certain degree between the resonant circuits, the impedances may be changed stepwise by turning the switches. In this manner, the control can get done relatively easily with high enough transfer efficiency maintained.
If an audio device and a loudspeaker need to be connected together wirelessly by the magnetic resonance method, one of the following two methods can be taken. According to one of the two methods, electric power is transferred between the two coils by the magnetic resonance method, an audio signal is transmitted and received wirelessly over a wireless LAN, for example, and the electric power is received at the loudspeaker end and used to drive the circuits, thereby reproducing the audio signal received.
The other method is transmitting wirelessly both the electric power to drive the loudspeaker and the audio signal between the coils at the same time by the magnetic resonance method. According to such a method, the electric power to drive the loudspeaker is transmitted wirelessly between the resonant circuits on the transmitting and receiving ends by using a frequency signal (a periodical signal), of which the frequency is adjusted to the resonant frequency of the resonant circuits, and the frequency signal is subjected to either AM or FM modulation according to the audio signal and then transmitted to the loudspeaker. Such a technique is disclosed in Japanese Patent Application Laid-Open Publication No. 2009-153056 (which will be referred to herein as “Patent Document No. 2” for convenience sake), for example. In such an arrangement, the circuits on the loudspeaker end may all be passive circuits, and the drive power can be supplied to the loudspeaker and the audio signal can be read by demodulating the AM/FM modulated signal.
Meanwhile, Japanese Patent Application Laid-Open Publication No. 2007-49437 (which will be referred to herein as “Patent Document No. 3” for convenience sake) discloses a method for transmitting both electric power and an audio signal to a loudspeaker at the same time not by the wireless power transmission method but through a cable by superposing a PWM modulated signal, representing the audio signal, on a DC power signal. According to this technique, the PWM modulated audio signal is superposed on the output of a power supply circuit and transmitted to the loudspeaker, and then demodulated on the loudspeaker end, thereby reproducing an audio signal that has the electric power to drive the loudspeaker. If the volume needs to be adjusted, the amplitude and the pulse width of the PWM signal just need to be changed on the transmitting end.
In transmitting electric power and an audio signal at the same time by the magnetic resonance method, however, if the distance between the audio device itself and the loudspeaker changes, the audio signal could not be obtained as intended. The present inventors confirmed via experiments that particularly when the transfer efficiency was increased by controlling the impedances as described above, the audio signal thus obtained often had problems.
It is therefore an object of the present invention to provide an audio system that can transmit electric power and an audio signal to a loudspeaker wirelessly by electromagnetic resonance method and that can read the audio signal with high fidelity. Another object of the present invention is to provide a device on the transmitting end and a loudspeaker for such an audio system.